The present invention relates to the field of hydraulic units, including pumps and motors. In particular, this invention relates to a device for attenuating vibration and periodic pressure fluctuations in hydraulic units having variable displacement controlled by a servo system which is hydraulically coupled to the periodic portion of the output pressure of the unit. The device is particularly useful on axial piston units.
Vibrations and pressure fluctuations are commonplace in hydraulically operated equipment. However, end users are becoming increasingly concerned about and intolerant of the contribution of hydraulic units to the overall levels of vibration, pressure fluctuation and noise on their machines. Heretofore it has been difficult to significantly reduce the pressure fluctuations and vibrations in hydraulic units, particularly in axial piston pumps and motors where a portion of the output power (in terms of flow and pressure) is used as a power supply for a control system utilizing one or more servo pistons to vary displacement. High levels of vibration and pressure fluctuation result from the unsteady component of the output pressure which is periodic in nature. This unsteady component of the output pressure is typically present and is seen at the piston frequency and may include one or more harmonics of this frequency. High levels of vibration and pressure may also occur if a separate power supply for the control system is dynamically coupled to the output characteristic of the hydraulic unit.
The unsteady pressure of the supply oil that reaches the servo causes the swashplate to oscillate at the primary forcing (piston) frequency. Swashplate oscillations will affect the unsteady or periodic component of the output pressure in a closed feedback loop. A resonant condition will develop if the gain of the closed feedback loop is sufficiently high and the phase relationship is shifted 180 degrees from the ideal; the amplitude of the oscillations grows to a significant value.
The resonant frequency is a function of the geometry of the porting and the compression/decompression dynamics of the pistons in the hydraulic unit. Additionally, there are transport lags due to the time it takes for the signal to reach the control valve. An additional phase lag takes place as a result of the restriction of the control valve and the volume of the servo piston.
Therefore, a primary objective of the present invention is the provision of an apparatus for attenuating pressure fluctuations and vibrations in hydraulic units. To attenuate something is defined as reducing its intensity.
Another objective of this invention is the provision of an apparatus for attenuating pressure fluctuations and vibrations in axial piston pumps and motors where the output power is used as a power supply for the control system or a separate power supply is dynamically coupled with the control system.
Another objective of this invention is the provision of an apparatus for attenuating pressure fluctuations and vibrations that can be used to retrofit existing hydraulic units.
Another objective of this invention is the provision of an attenuator that is economical to manufacture and durable in use.
These and other objectives will be apparent to one skilled in the art from the description which follows.